The present invention relates to viscous fluid couplings, and more particularly, to such couplings which are used to drive vehicle radiator cooling fans, wherein the engagement or disengagement of the viscous fluid coupling may be controlled in response to a remotely sensed condition, such as coolant temperature.
A viscous fluid coupling (viscous fan drive) of the general type to which the present invention relates is illustrated and described in U.S. Pat. No. 3,055,473, assigned to the assignee of the present invention, and incorporated herein by reference. A typical viscous coupling receives input drive torque from a vehicle engine, and transmits output drive torque to a radiator cooling fan. The conventional viscous coupling includes an output coupling defining a fluid chamber, valve means operable to separate the fluid chamber into a reservoir chamber and an operating chamber, and an output coupling rotatably disposed in the operating chamber and operable to transmit input drive torque to the output coupling in response to the presence of viscous fluid in the operating chamber. The valve means includes a valve member moveable between a closed position blocking fluid flow into the operating chamber, in an open position permitting fluid flow into the operating chamber.
In certain vehicle applications, it has become desirable to sense directly some parameter of the vehicle, such as the temperature of the liquid coolant entering the radiator ("top-tank" temperature), and to control the viscous fan drive in response to changes in that parameter. One benefit of the arrangement described is that the responsiveness of the fan drive is improved, when compared to the earlier, prior art fan drive which was responsive only to sensed ambient air temperature. Accordingly, the conventional fan drive described above has been modified by the addition of an actuator means operable to move the valve member between the closed position and the open position in response to changes in an input signal. Such a "remote sensing" viscous coupling is illustrated and described in U.S. Pat. No. 5,152,383, assigned to the assignee of the present invention and incorporated herein by reference.
Viscous fan drives have been extremely successful commercially for many years. However, in the course of development, testing, and operation of viscous fan drives (whether of the ambient temperature sensing type, or of the remote sensing type), there are several operating situations in which the prior art viscous fan drives have not responded adequately.
One of these operating situations is referred to as the "stoplight idle" condition. When a vehicle equipped with a conventional viscous fan drive comes to rest, for example, at a traffic signal, engine speed falls below the "demanded" fan speed, i.e., the fan speed necessary to cool the engine adequately. Of course, in a conventional fan drive installation, the fan speed can never exceed the input speed (engine speed multiplied by the pulley ratio). For a remote sensing clutch with classic (prior art) feedback control, the fan drive logic will, therefore, move the valve member toward a fully open position, filling the operating chamber with fluid in a vain attempt to reach the "demanded" fan speed. A similar result occurs with ambient air sensing type clutches. In a stop light idle condition, a bimetallic control element will either not change the position of the partially open valve, or will actually move the valve further toward the open position. This is caused by heated air dissipated by the vehicle engine.
Unfortunately, when the vehicle accelerates from the stopped condition at the traffic signal, the fan drive operates in a fully engaged condition, when such is not really necessary, resulting in excessive fan noise as the input speed to the fan drive increases. This undesirable noise continues until enough fluid is pumped from the operating chamber to the reservoir chamber to bring the fan drive down to the then-current demanded fan speed.